1. Field of the Present Disclosure
This disclosure relates generally to a geometrical measuring technology, and a device according to the present invention serving for the indirect determination of the position of points with the help of two trackers, wherein there is no need for a direct line of sight between the point position to be determined and the trackers.
2. Description of Related Art
A tracker includes means for generating a light beam, in particular a laser beam. With the help of a mirror, the laser beam is directed to a target point, from where it is reflected back into the tracker by a retroreflector. The tracker further includes means for automatically aligning the mirror such that the path of the reflected beam, if at all possible, is the same as the path of the emitted beam. These means allow the tracker to follow a moving target point with the mirror or with the laser beam respectively. The orientation of the mirror is recorded as a measured value representing the direction from the tracker to the target point. The tracker even further includes an interferometer for recording distance changes between the tracker and a moving target point (relative distance measurement). If so required, the tracker also comprises means for an absolute distance measurement, which means are, for example, based on the Principle of Fizeau.
Trackers are primarily used for recording the track of a moving target point or for scanning surfaces, wherein a target point is moved over the surface. However, trackers can also be used to determine the position of stationary target points, wherein these positions are defined by the (known) tracker position, by the orientation of the tracker mirror (direction of the tracker beam), and by the absolute distance between tracker and target point. For direct measurements of this kind, a direct line of sight acquisition between the target point and the tracker is needed, i.e., the tracker beam must not be interrupted between the tracker and the target point.
It would be desirable when using a tracker for determining point positions or track points to be able to do so without direct line of sight connection from the tracker to the point or track. If this were possible, the installation of a multitude of trackers or the displacement of a single tracker could be avoided in many instances.
For determining point positions using a theodolite or a digital camera (photogrammetry), tools for indirect measurement are used if there is no direct line of sight connection from the measuring instrument to the point. Such a tool comprises a reference point and at least two target points having exactly defined positions relative to each other. The tool is designed to be positioned such that the position of the reference point relative to a point position to be determined is exactly defined. For determining the position of a specific point, the tool is positioned such that the named relationship between the reference point and the specific point is established and such that between the target points of the tool and the measuring instrument there is a direct line of sight connection. Thereupon, the positions of the target points are determined and from the known positions of the target points relative to one another the position and at least partially the orientation of the tool are determined. From the calculated position and orientation of the tool, the position of the reference point—and from the known relative positions of reference point and sought point, the position of the sought point is calculated. It is also known to use such a correspondingly equipped tool for determining not only a point position but also the orientation of a surface on which this point is located.
The relative positions of the reference point and the point to be determined are defined purely mechanically, for example with a feeler, a tip or a pin with shoulder, or else opto-mechanically, for example through a virtual point, a reticule plate or cross-hairs. The arrangement of the target points usually represents a simple geometrical shape (straight line, circle, sphere), because such a shape can be mathematically described in a very simple manner. Using the high data processing capacities available today more complicated arrangements are also applicable.
Tools for determining point positions, to which no direct line of sight connection exists, and/or for determining surface orientations are described, for example, in the publications SE456454 or WO91/16598. Publication DE4038521 also describes tools of the described type, which tools comprise two retro-reflectors positioned together with the reference point in one straight line. The distances and directions of the two retro-reflectors from the measuring instrument (tachymeter) are measured after each other. In the same publication it is suggested also to replace the two retro-reflectors by only one reflector and to displace this one reflector from a first into a second position between the two successive measurements and this technique is further described and elaborated in Markendorf, et al U.S. Pat. No. 6,675,122 which document is incorporated herein by reference.
The tools for the indirect determination of point positions and/or of surface orientations as described above can be used together with a tracker, for example, for determining the position of a point from which there is no direct line of sight connection to the tracker. For this purpose, the target points of the tool have to be designed as retro-reflectors. For measuring, the tracker is directed at one of the target points of the tool, the direction and the absolute distance from the tracker to the aimed at target point is determined, and this procedure is repeated for all target points of the tool. The measured directions and distances are then processed in a well known manner.
This procedure is possible, but rather elaborate, because the tracker has to be aimed at every single target point and this has to be done by hand unless there are special aids installed, e.g. a digital camera functionally connected with the tracker or means for enlarging the field of vision of the tracker. Furthermore, for the measurement of every target point an absolute distance measurement has to be carried out, a measurement, which due to the means as available today is substantially less accurate than the interferometric, relative distance measurement. However, the elaborate procedure as described above can be avoided if the target points are arranged on the tool so close together that all of them appear in the very narrow field of view of the device without changing the device direction. This, on the other hand, restricts the accuracy considerably.
Markendorf, et al, U.S. Pat. No. 6,675,122, discloses an apparatus, and now referring to the figures of this patent, for indirect determination of point positions and/or of surface orientations, where in point positions a laser tracker (1) adapted for relative and absolute distance measurement is used. A measuring device is utilized, the device including a target point (retro-reflector 3) for the laser beam of the tracker, the target point being movable along a trajectory path (A). The position and orientation of the trajectory path (A) of the target point is precisely defined relative to a reference point (R) of the device (2) and the device (2) is positionable in the region of a point position (P) to be determined such that the position of the reference point (R) relative to the point position (P) to be determined is precisely defined. For determining the point position (P), the device (2) is positioned in the region of the point position (P) to be determined, the target point is positioned in a starting position (3.1) and is detected by the tracker (1) by direction determination and absolute distance measurement. Then the target point is moved along the trajectory path (A) being followed by the tracker (1), while direction and distance changes are recorded. In comparison with an indirect determination of a point position with the help of a known measuring tool having a plurality of stationary target points for the tracker beam, the determination by means of the target point moving along the trajectory track (A) is more simple and more accurate. In Gehter CN1194688A a geodesic measuring staff with a bar code graduation has a body with a cross-section that is substantially symmetrical about an axis. The elements that make up the bar code graduation form closed lines on the outer surface of the measuring staff and lie in a cutting plane perpendicular to the longitudinal axis of the measuring staff. Stephen et al US2004136012 discloses a measuring system comprising a measuring device (1) with a laser tracker (2) and an opto-electronic sensor (3) having fixed positions relative to one another, a system computer and an auxiliary measuring tool (4) with a reflector (5) and at least three light spots (6), is calibrated with the following calibration steps: The auxiliary measuring tool (4) is rigidly coupled with an arrangement of auxiliary reflectors (5′) and is moved around at least two different rotation axes. In at least two rotation positions relative to each one of the at least two rotation axes, reflector (5) and auxiliary reflectors (5′) are registered by the laser tracker (2) and the light spots (6) are registered by the opto-electronic sensor (3). From the measured data of the laser tracker (2) positions and orientations of the reflector arrangement relative to the laser tracker (2) and from the measured data of the opto-electronic sensor (3) positions and orientations of the light spot arrangement relative to the opto-electronic sensor (3) are calculated and from this the at least two rotation axes relative to the reflector arrangement and relative to the light spot arrangement are calculated. Then calibration data are calculated by equating corresponding rotation axes. For the measuring steps a calibration device (9) comprising a revolving table (11) and a wedge (12) installed on the table is used, wherein the auxiliary measuring tool (4) is mounted on the wedge (12) in two different orientations. Markendorf et al U.S. Pat. No. 6,667,798 discloses a system using a laser-tracker (1) with an interferometer together with a retroreflector (3) in the form of a triple prism or a triple mirror arranged on an object and designed for parallel reflection of the laser beam (5) directed to the reflector (3). The position of the object is computed by way of measurement data with respect to the direction of the laser beam (5) and its path length. Additionally the spatial orientation of the object (2) is determined by producing additional measurement data with respect to the angle of incidence (alpha) of the laser beam into the reflector (3) and/or with respect to an adjustable orientation of the reflector (3) relative to the object (2) and by computing the position and spatial orientation of the object (2) by way of measurement data with respect to the direction and the path length of the laser beam (5) and by way of this additional measurement data. Thereby the measurement data of the direction measurement and of the interferometric measurement may be corrected by way of the additional measurement data with respect to the angle of incidence (alpha) such eliminating their dependency on the angle of incidence. For determining the angle of incidence (alpha), the reflector (3) is designed in a manner such that a central part of the laser beam (5) passes through the reflector (3) unreflected and behind the reflector (3) impinges a position sensor (12) arranged stationary relative to the reflector, while a peripheral part of the laser beam (5) is reflected parallel and used for the interferometric measurement. Lindqvist WO9116598 discloses a device for determination of the topography of a surface by measuring its normal vector point-by-point, as well as devices for determination of the curvature of the surface in these points, and for determination of the orientation, position, shape and size of holes in the surface. These devices are based on the use of opto-electronic sensors (1; 1-2) for measurement of the spatial position of active light sources. The invention is furthermore based on an accessory tool (1; 4) consisting of a body (3a; 5), a minimum of two light sources (3a; 6-8) and three contact pins (3a; 10-12) or a plane contact surface. The contact pins or the contact surface positions the tool unambiguously onto a surface (1; 3). By knowledge of the positions of the light sources relative to the contact pins or contact surface of the tool, the orientation of this surface can be determined from the measured spatial coordinates of the light sources. Meier, et al U.S. Pat. No. 5,893,214 discloses a retroreflecting triple prism (17) within a measuring sphere whose base face (18) cuts out a part of the surface (11) of the measuring sphere and whose height is approximately equal to the radius of the measuring sphere (10), the center (16) of the measuring sphere (10) lying on the altitude (19) of the triple prism (17). Greenwood et al U.S. Pat. No. 5,920,483 discloses that large machines, especially those having working envelopes in excess of fifteen feet, exhibit unacceptable errors because of thermal expansion and mechanical misalignments between the axes. The invention uses an interferometric laser tracker or a comparable 3D position sensor to measure the position of a retroreflector attached to the end effector, e.g. a machine head when the machine comes to rest. A computer compares the measured position to the desired position according to the machine media, and adds the appropriate correction with trickle feed media statements to move the machine to the correct position prior to further machining.
Although the techniques described here and in the related art are powerful measuring methods, the prior art fails to teach a tool that has the stability necessary for extreme accuracy. The Markendorf, et al patent U.S. Pat. No. 6,675,122 for instance uses a retroreflector that is mounted for sliding movement on the tool. This introduces errors due to the mechanical mount and movement enabling elements. The present disclosure distinguishes over the prior art providing heretofore unknown advantages as described in the following summary.